1. Field of the Invention
The present invention generally relates to planing hulls for watercraft and, more particularly, to planing hulls for sailboards/windsurfers for improving the transition from displacement operation to planing operation and exhibiting increased speed over a wider range of wind speed.
2. Description of the Prior Art
Hulls of watercraft may be of either of two distinct types: a displacement hull which derives vertical lift from the weight of water displaced by the hull and a planing hull which derives vertical lift from thrusting water downwardly by the bottom surface of the hull when in motion. At rest or at low speed, planing hulls function in the same manner as displacement hulls. Displacement hulls are most efficient and derive greatest speed for a given amount of power if they are a long and narrow, streamlined shape. Planing hulls, on the other hand can be much more efficient than displacement hulls when planing and, since lift is derived from the angle of attack between the bottom surface of the hull and the water surface, are most efficient if wide and short; directly conflicting with the preferred shape for displacement hulls.
Therefore, in general, the more fully a hull is optimized for planing efficiency, the more power is required to reach planing speed. It follows that planing hulls must often represent a compromise between efficiency in the displacement and planing modes of operation, particularly where available motive power is limited such as when sails are employed. Conversely, wind/sail-powered watercraft such as sailboards generally operate well only within a narrow range of wind conditions.
For example, commercially available sailboards such as the Mistral Ultralight and the F2 race board are made for non-planing or marginal planing conditions and are long, narrow and streamlined but, as would be expected, do not plane well and are not as fast as planing xe2x80x9cslalomxe2x80x9d or short boards. For example, some boards like the commercially available Pro-Tech C. A. T. are wide and short and very fast when planing but comparatively slower at displacement operation speeds in light winds. Such short boards are also somewhat more difficult to control and xe2x80x9cunfriendlyxe2x80x9d to inexperienced wind surfers. Other boards which are short and narrow are fast when planing because they achieve the proper attack or planing angle but require more wind to achieve planing.
Other factors in board design also affect performance in a variety of conditions, particularly in regard to planing. For example, if a board is flat, it will plane in lower wind but tends to ride xe2x80x9chardxe2x80x9d under conditions of even a slight chop (e.g. wind driven small waves) If it is large so that it planes in low wind, it is not as fast in higher winds because it will assume too small an angle of attack. If the bottom of the board has a V-shape, it will ride more smoothly but will not plane as fast (e.g. requires more wind to achieve planing). The board will also ride more smoothly if it has more xe2x80x9crockerxe2x80x9d (e.g. curvature front-to-rear). It will be faster when not planing and may be faster when planing in high wind due to reduction in wetted area. However, increased xe2x80x9crockerxe2x80x9d makes it plane more slowly and requires additional wind for planing due to the decreased angle of attack at the rear which may even cause suction where the bottom surface tries to leave the water. Thus, increased rocker is generally desirable in displacement hulls while decreased, if any, rocker is desirable in planing hulls.
Commercially available boards which are designed primarily to perform in light wind are generally too flat to perform well in higher wind. Such boards are more flat and plane at an angle of attack less than the optimum 40xc2x0-70xc2x0; thus having increased wetted surface and associated drag.
In this regard, it is known for relatively small motor boats (having a significant degree of rocker) to install trim plates extending behind the transom or stern of the boat which can be deflected slightly downwardly to provide lift at the stern of the boat and thus increase the stern angle of attack when the hull is beginning to plane. The trim plates thus reduce power requirements and smooth the transition between displacement and planing modes of operation. However, it is not practical to use such expedients on a sailboard since control by the operator is impractical.
Further, for both boats and sailboards, such trim plates or hull shaping to the same purpose (which is effectively contrary to the function of rocker), if not properly set for the current speed, can cause an effect known as porpoising. Porpoising is an unstable state in which excess lift at the rear or stern forces the bow lower in the water where rocker causes increased lift at the bow; resulting in an oscillatory pitching action and increased drag. Moreover, with sailboards, some of the deleterious effects of excessive rocker, such as increased angle of attack can be ameliorated by alteration of fore and aft balance at the displacement/planing transition by a suitably skilled operator.
Planing hulls may also be of either the stepped or unstepped types. While the latter has a substantially continuous lower surface, the former, stepped type has an upward step or recess in the bottom surface which is either in front of the center of gravity or very small. This step, under planing conditions at relatively high speed, reduces the wetted surface and associated drag. However, the discontinuity in the shape of the bottom surface also tends to increase drag (for reasons that have not previously been well-understood but intuitively thought to be related to a combination of turbulence and suction behind the step and deeper extension into the water) during displacement mode operation and increase the difficulty of the transition between displacement and planing conditions as well as increasing the power/speed required to reach planing conditions.
Possibly for this reason, stepped bottom surfaces are not generally used for sailboards. Among currently commercially available designs, only the Pro-tech C. A. T., which has an approximately one-half inch step near the rear of the board, provides a stepped bottom surface rather than a single running or planing bottom surface. Further, the step is either completely surrounded by water (during displacement operation) so it only functions as a step in the mainly displacement mode (low speed planing or slower) or completely out of the water (during planning operation).
In summary, while numerous design features of watercraft hull shapes are known for enhancement of efficiency and performance, each such feature and most combinations thereof have tended to narrow the range of conditions under which such enhancement can be realized. These limitations are particularly critical where available power is limited as is the case with sailboards which operate solely under sail power and where the sail area is severely limited by the necessity of being held in place by a human operator, principally by balancing wind force with limited body weight.
Further, good planing performance is of high importance with sailboards since high speed is very desirable in the windsurfing sport and less power is required while planing, as alluded to above. Moreover, the speed increase which occurs when planing is achieved greatly increases apparent wind speed during reaches (sailing generally across or toward the wind), allowing substantial increase in the speed attainable as well as generally increased maneuverability. Nevertheless, known designs of sailboard hulls only support such levels of performance within a limited range of conditions (e.g. wind speed, water surface chop, and the like) while the cost and size of sailboards and other practical considerations effectively prevent alternative use of sailboards of different designs to exploit particular conditions which may prevail at any given time. Nevertheless, known designs of sailboard hulls only support such levels of performance within a limited range of conditions (e.g. wind speed, water surface chop, and the like) while the cost and size of sailboards and other practical considerations effectively prevent alternative use of sailboards of different designs to exploit particular conditions which may prevail at any given time.
It is therefore an object of the present invention to provide a hull design, particularly for a sailboard, which has high planing performance and can reach a planing mode of operation over a wide range of wind speed.
It is another object of the invention to provide a hull design which has a stable and consistent angle of attack when planing over a wide range of wind speed.
It is a further object of the invention to provide a sailboard hull having a stepped lower surface that reduces difficulty of the transition from displacement to planing operation and avoids other observed undesirable effects such as increased drag during displacement operation.
In order to accomplish these and other objects of the invention, a sailboard apparatus and hull thereof is provided including a first planing surface, a second planing surface extending below the first planing surface and forming a step at a juncture of the first planing surface and a rear portion of the second planing surface, and an arrangement for reducing suction behind the second planing surface including, for example a vent to or through the top surface of the sailboard hull or to the side of the sailboard hull above the surface of water supporting the sailboard hull or streamlining of the second surface.